1. Field of the Invention
The present invention relates to a polarization-multiplexing optical transmitter, a polarization-multiplexing optical receiver, a polarization-multiplexing optical transceiving system, and a controlling method thereof useful for transmission using polarization multiplexing, and, more particularly to a polarization-multiplexing optical transmitter, a polarization-multiplexing optical receiver, a polarization-multiplexing optical transceiving system, and a controlling method thereof that can stably split polarization components.
2. Description of the Related Art
For implementing an ultra-high speed optical transmission system of more than or equal to 40 gigabits per second (Gbit/s), a polarization multiplexing technology has attracted considerable interest. In the polarization multiplexing technology, two polarization states that are orthogonal to each other are available in a same wavelength signal and by using the two polarization states or components, data of two interdependent signals is transmitted. By using the polarization multiplexing technology, a modulating speed can be reduced to half. A technology related to the transmission system using the polarization multiplexing technology is disclosed in Japanese Patent Application Laid-open No. S62-024731.
The polarization states of polarization-multiplexing signals receive effects such as bending, birefringence, and oscillation of a transmission optical fiber. Due to this, when signals are reached to a receiver, uncertainty and temporal variation of polarization states are accompanied with the signals. Thus, for stably splitting polarization components on a receiving side, a specific adaptive control is required. In the transmission system disclosed in Japanese Patent Application Laid-open No. S62-024731, for detecting whether polarization splitting is in an appropriate state, a control bit-string that is not related with input data is additionally included with respect to a bit sequence transmitted to two polarization channels. However, on the receiving side, adding a function that performs a complex signaling process is required. Due to this, a structure of a transmitter becomes complicated. Furthermore, by adding the control bit-string, because a mark ratio of each polarization channel varies from half, optimization of bit error rate (BER) characteristics of the receiver becomes complicated. Further, because transmission is to be carried out in a bit rate higher than the bit rate of the input data, the structure of the receiver also becomes complicated.